The underlying method of optical absorption spectroscopy is generally known and is also used in various arrangements for determining the concentration of substances. The light sources used for these systems include broadband light sources, such as thermal emitters and various kinds of gas-discharge lamps (EP 0 656 535 A1), and narrow-band laser light sources (Lambrecht, A. and J. Koeth, Quantum cascade laser—a new laser light source for optical analytical measuring. Technisches Messen, 2005). Recently, LEDs have also been used as spectrally selective light sources for substances with mainly comparatively broadband absorption characteristics (DE 10 2008 064 173 A1). However, many substances have comparatively narrow-band absorption characteristics relative to the spectral bandwidth of the LED light source, such as for example dissolved benzene (generally designated hereinafter as “narrow-band”, and the opposite case as “broadband”).
In absorptiometry, high measurement accuracy for determining the concentration of substances in fluid media requires high raw signal resolution and raw signal stability, which is not directly achievable based on the emission stability of conventional light sources used in spectroscopy. Emission stability means an emission spectrum that is stable with respect to the selected measurement wavelengths for the measurement time, and amplitudes of these wavelengths that are stable for the measurement time.
Determination of the concentration of substances is based on measurement of the attenuation of light, caused by the light absorption of the substance at a specified concentration. Conventionally, steps are taken for referencing the emission properties of the light source, so as to compensate source-induced disturbing effects with respect to emission stability in the measurement signal. In conventional arrangements, for this purpose the light is for example divided between source and optical measuring path and partly directed onto a reference detector. Therefore in addition to the increased technical expense for the optical beam guidance, two receiving units working exactly identically are also required. The resultant measuring systems are therefore often very complex and of intricate mechanical construction and therefore also of high cost. This applies in particular for example to systems based on broadband light sources and the receiving units often used here, based on a spectrometer.
Another solution in conventional systems is based on mechanical switches in the optical beam path. In this way it is possible to perform referencing of the light used for measurement with just one detector unit. In this case, however, there are high requirements on the reproducibility of the switching operation, to ensure spectral and amplitude stability. Continuous determination of the concentration of substances with high resolution is not possible here, owing to the time taken for referencing.
Changes in the transmission of the absorption measuring path or of the reference measuring path and changes, for example ageing, of the detector unit(s) have a direct influence on measurement accuracy. This necessitates regular, cyclic recalibration of the whole system.
Another basic possibility for achieving high measurement accuracy in absorption measurements consists of utilising the wavelength-specific absorption of the substance, so as to compensate amplitude fluctuations of the source, of the optical path and of the detectors. However, this requires high spectral resolution of the measuring system and therefore a high level of instrumentation and the associated high costs, for example the use of very-high-resolution spectrometers, with a spectral resolution of typically 0.01 nm or less.
The use of narrow-band laser light sources for laser absorption spectroscopy is, along with the arrangements described already based on broadband light sources, another method often used for determining the concentration of substances in fluid media. An exemplary embodiment of this is TDLAS (tunable laser absorption spectroscopy). In systems corresponding to this method, the emission wavelength of a suitable laser light source is modulated spectrally for example by means of the current the temperature [sic]. In this way, an absorption peak of the substance to be measured is sampled at different wavelengths. By comparing with known spectra, disturbances can largely be suppressed and the concentration of substances can be determined.
High-resolution concentration measurements are performed using this method. An important drawback of this measurement technique is the availability of economical laser light sources for certain wavelengths. Therefore this method is only available for selected substances and is, for example when using quantum-cascade lasers, extremely expensive. This method of measurement is, in addition, restricted in the measuring rate owing to the time-dependent wavelength modulation of the source, and there is no continuous determination of concentration.
To summarise, from the prior art, for high-resolution determination of the concentration of substances with primarily narrow-band absorption characteristics (for example benzene, toluene), there are the following disadvantages: conventional broadband light sources must be referenced both with respect to the total emission intensity and with respect to the spectral distribution of the emission intensity. Possibilities for referencing that may be considered require a certain proportion of light and/or measuring time and therefore reduce the signal/noise ratio of the measurement signal and/or the temporal resolution of the measurement signal (measuring rate). Each referencing is basically associated with a not inconsiderable expenditure of effort and associated costs and for example increases the complexity of the whole system and correspondingly also the vulnerability of the system.
Generally, devices based on broadband light sources or lasers, compared to LED-based systems, are as a rule much more expensive, less mechanically robust, more prone to interference and are comparatively large and have higher power consumption. The electronic controls for LED-based measuring systems are also much less expensive.
The known LED-based absorption spectroscopy (DE 10 2008 064 173 A1) is not, however, very suitable for high measurement accuracy in particular for substances with very narrow-band absorption characteristics owing to the resultant small absorption cross-section. The resultant absorption cross-section of a measuring system, consisting of the emission characteristics of the LED and the absorption property of the substance to be determined, is found from the attenuation of the transmission signal in the system. In this connection, FIG. 1 shows a comparison of the resultant absorption cross-section for benzene or NO2 for an LED-absorption arrangement. As noted below, FIG. 1a shows the example of narrow-band absorption characteristics of benzene. The resultant absorption is much less than the peak absorption. FIG. 1b shows the example of broadband absorption characteristics of NO2. The resultant absorption is similar to the peak absorption.
In addition, the necessary referencing requires a proportion of the light output that is not negligible. Especially in the case of measuring tasks in the mid-infrared or in the UV, the luminous efficiency of the LED sources is generally not very high and the light output available for the measurement has a decisive influence on the resultant measurement accuracy and resolution of the measurement signal.